1. Field of the Invention
The present invention relates to an image forming apparatus for reducing jaggies on edges in image data that has undergone halftone processing, and an image forming method therefor.
2. Description of the Related Art
Conventionally, several techniques for reducing jagged edges, called “jaggies”, occurring around edges of characters or the like in an image forming apparatus have been proposed. There are various reasons as to why jaggies occur, and it can be thought that there are roughly two types of jagged edges, namely, jagged edges of pixels due to a low-resolution printer and jagged edges due to halftone processing such as screen processing.
As one example of a technique for reducing the former, there is a technique for detecting edges from a binary image through pattern matching and adding or removing pixels to or from places where pixels match, corresponding to the pattern (see Japanese Patent Laid-Open No. 10-42141, for example). With this technique, smoothing processing is performed on edges by detecting places where jaggies occur through pattern matching and dividing a single pixel into multiple pixels in the case of a binary printer and adding halftone dots in the case of a multi-value printer.
Also, one example of a technique for reducing the latter is a technique for generating correction data from image data prior to halftone processing and adding the correction data to edges in the image data that has undergone halftone processing such that the edges are rimmed with the correction data (see Japanese Patent Laid-Open No. 2006-295877, for example). This technique reduces jaggies due to screen processing by determining whether or not each pixel is an edge pixel that requires smoothing processing, and if the pixel is an edge pixel, comparing the correction data and the image data that has undergone halftone processing and outputting the one with the greater value. With this technique, the former jagged edges of pixels due to a low-resolution printer can also be reduced, irrespective of a simple configuration.
However, although jaggies due to screen processing can be reduced with the above-described conventional method, the density of edges ends up simply increasing because, out of the correction data and the image data that has undergone halftone processing, the one with the greater value is output for edges. This phenomenon is not a great problem with original documents in which characters, lines or the like have regular edges at a uniform density, but it can cause image interference, such as a great change in coloration, with original documents such as a natural image that include irregularly packed edges.